Various industrial production processes of acetic acid have been known. Among others, an industrially excellent process includes a process which comprises continuously allowing methanol to react with carbon monoxide with the use of a metal catalyst (such as a rhodium catalyst) and methyl iodide in the presence of water to give acetic acid. Moreover, recently improvement in reaction conditions and catalysts was investigated, and an industrial process for producing acetic acid with a highly efficient production has been developed by addition of a catalyst stabilizer (such as an iodide salt) and the reaction under a low water content condition compared with the conventional condition.
In this process, a volatile component is separated when acetic acid is separated from a liquid reaction mixture by distillation. The volatile component contains a useful component such as methyl iodide, while the component is a liquid component containing acetaldehyde. Therefore, the volatile component is collected or recycled to the reaction system after acetaldehyde is separated by distillation (or condensation). The volatile component contains an acidic component (such as hydrogen iodide or acetic acid) in addition to methyl iodide, acetaldehyde, water, and methyl acetate. When hydrogen iodide or acetic acid is condensed (or produced) in an acetaldehyde distillation column under a distillation condition (such as an applied pressure or a high temperature), the corrosion of the acetaldehyde distillation column may be precipitated. Moreover, when the solution containing methyl iodide is subjected to distillation and then the separated fraction and/or the residue are recycled to the reaction system, the corrosion of a pump for recycling (recycle pump) or a line for recycling may be precipitated. Therefore, it is preferable that the concentration of the acidic component (such as hydrogen iodide or acetic acid) in the distillation column for separating acetaldehyde be reduced.
Japanese Patent No. 3581725 (JP-3581725B, Patent Document 1) discloses a recycling process which comprises carbonylating methanol and/or methyl acetate in a reaction medium containing a group 8 metal catalyst of the Periodic Table and methyl iodide; separating a volatile phase and a low-volatile phase from the carbonylation product, where the volatile phase contains the product, unreacted methanol and/or methyl acetate, and methyl iodide, and the low-volatile phase contains the group 8 metal catalyst; distilling the volatile phase to give an overhead containing the product, the unreacted methanol and/or methyl acetate, and methyl iodide; and recycling the overhead to the carbonylation reactor; wherein the overhead is a mixture containing acetaldehyde and methyl iodide, the overhead is recycled to the reactor after distillation of the overhead in the presence of methanol at a column top temperature of less than 55° C. and a reflux tank temperature of less than 25° C. and dissolution of acetaldehyde produced in a paraldehyde or metaldehyde form in a mixed solution containing methyl iodide and methanol in a weight ratio (methyl iodide/methanol) of 5/4 to 1/2 as a composition of a bottom fraction of the distillation column for removal or separation.
Incidentally, according to the process described in the document, use of methanol in the distillation column aims at dissolution of paraldehyde or metaldehyde and is not intended for inhibition of hydrogen iodide production. Moreover, the process described in the document needs methanol 0.8 to 2 times as much as the weight of highly rich methyl iodide contained in the overhead, and it is necessary to treat a large quantity of a liquid object to be treated composed of the total amount the overhead and methanol. Thus it is necessary to use a distillation column having a large column diameter, which is not economical.
WO2008/016502 publication (WO2008/016502, Patent Document 2) discloses a process for decreasing an aldehyde impurity from an acetic acid stream, which comprises allowing an acetic acid stream containing an aldehyde impurity to react with a hydroxy compound (such as glycol, polyol, or a C4-10 alcohol) to convert the aldehyde impurity into an acetal, and separating the acetal. Specifically, the document discloses the following recycling technique: an overhead containing methyl iodide, methyl acetate, acetic acid, water, and an aldehyde impurity is liquefied in a decanter, 5 to 50% of the resulting heavy phase (organic phase), which contains methyl iodide and the aldehyde impurity, is treated with a hydroxy compound in a ratio of 1 to 10 equivalents of the hydroxy compound relative to the aldehyde impurity and then distilled to separate an acetal fraction and a methyl iodide fraction, and the acetal fraction is wasted and methyl iodide is recycled to the heavy phase in the decanter or the carbonylation reaction. The document also discloses that methyl iodide to be used for the reaction can be produced by adding hydrogen iodide to a carbonyl reactor. Thus the document intends the hydroxy compound is used for conversion of an aldehyde into an acetal, and an acidic ion exchange resin is used for acetalation, and the document never intends the decrease in hydrogen iodide or acetic acid.
Japanese Patent Application laid-Open No. 2007-526308 (JP-2007-526308A, Patent Document 3) discloses a process for producing acetic acid, comprising the steps of: (a) reacting carbon monoxide with at least one reactant selected from the group consisting of methanol, methyl acetate, methyl formate, dimethyl ether and mixtures thereof in a reaction medium comprising water, methyl iodide, and a catalyst to produce a reaction product comprising acetic acid; (b) performing a vapor-liquid separation on the reaction product to provide a volatile phase comprising acetic acid, water, and methyl iodide and a less-volatile phase comprising the catalyst; (c) distilling said volatile phase to produce a purified acetic acid product and a first overhead comprising water, methyl acetate, and methyl iodide; (d) phase-separating the first overhead to provide a first liquid phase comprising water and a second liquid phase comprising methyl iodide; (e) adding dimethyl ether to the process in an amount effective to enhance separation of the first overhead to form the first and second liquid phases; and removing acetaldehyde from at least one of the first and second liquid phases, wherein the dimethyl ether is added to a stream associated with the acetaldehyde removal step. According to the document, dimethyl ether is used as a component to easily separate the first and second liquid phases, and the decrease in hydrogen iodide or acetic acid is not intended. Moreover, the document is silent on an amount to be added of dimethyl ether.
Japanese Patent Application Laid-Open No. 2000-72712 (JP-2000-72712A, Patent Document 4) discloses a process for producing acetic acid, which comprises a first step for allowing carbon monoxide to react with methanol, dimethyl ether, or methyl acetate in the presence of a rhodium catalyst, an iodide salt, and methyl iodide; a second step for distilling the liquid reaction mixture obtained in the first step to separate a high-volatile phase containing a carbonyl compound and a low-volatile phase; a third step for distilling the high-volatile phase containing the carbonyl compound obtained in the second step to separate a product containing acetic acid and an impurity containing the carbonyl compound; a fourth step for allowing the impurity containing the carbonyl compound obtained in the third step to contact with water to separate an organic phase containing an alkyl iodide and an aqueous phase containing the carbonyl compound; and a fifth step for sending back the organic phase obtained in the fourth step to the reaction step; wherein the contact of the impurity containing the carbonyl compound with water in the fourth step is carried out at 30 to 60° C. The document discloses that the process may comprise a 3b step, between the third step and the fourth step, for distilling the impurity containing the carbonyl compound obtained in third step by a multistage distillation column and that methanol 0.1 to 55 mol times as much as the iodide ion existing in the multistage distillation column may be fed in the distillation.
Moreover, the document discloses that (i) in the 3b step, hydrogen iodide produced by the reaction of methyl iodide with water sometimes causes corrosion of a metal used for the distillation column, (ii) since the reaction is an equilibrium reaction, addition of methanol to the distillation column inhibits the production of hydrogen iodide and then the metal corrosion, and (iii) since methanol is a lower boiling point component and the azeotropic temperature of hydrogen iodide and water is 127° C., methanol is preferably added at the bottom or vicinity thereof of the distillation column. Further, the document discloses in Examples that, in the 3b step, methanol 10 mol times as much as the iodide ion concentration in a 80-plate distillation column is added at 10 g/hr at a bottom gaseous phase in the distillation column, and the distillation is continued at 82° C., and accordingly the concentration of the iodide ion in the multistage distillation column was not more than 1 ppm; and that addition of a predetermined quantity of methanol to a liquid mixture as a model solution containing methyl iodide, water, and hydrogen iodide reduced the concentration of the iodide ion.
In the process described in the document, however, a liquid mixture containing only methyl iodide, water, and hydrogen iodide is used as the model solution, and the effect of methanol added on a model solution containing acetic acid and methyl acetate is not investigated. Moreover, the feeding of methanol about 0.1 to 55 mol times as much as the amount of the iodide ion is insufficient to decrease the acid concentration in an actual process solution containing not only hydrogen iodide but also acetic acid or methyl acetate and having a complicated composition. Further, according to the process described in the document, since methanol is added to the bottom gaseous phase, it is difficult to efficiently inhibit the corrosion of the whole distillation column. Thus the process is not efficient.